The present invention relates to printed boards and methods and processes of manufacturing same particularly three dimensional printed circuit boards. Currently, the art of printed circuit board manufacturing involves both subtractive and additive techniques. The subtractive processes are currently the most popular PC manufacturing technique and it involves a PC board which is entirely coated with copper. The holes are drilled and sensitized and electroless copper flashes used. The PC board artwork is used then to construct an acid-resistant shadow mask on the copper so that when submerged in acid the copper is eaten away leaving a pattern of conductive wires in the copper. This conductive pattern is the boards printed circuit.
In the additive process, the acid etching aspect is eliminated and this allows the manufacturers to improve their circuit board density and product yield. See Chapter 16 of the text "The Design and Drafting of Printed Circuits" by Darryl Lindsey, 1979, 1984, Published by Bishop Graphics entitled "Manufacturing Process of PC Boards".
In either system, a number of chemicals and chemical byproducts are used and/or generated, which creates problems when they cannot be reused and must be stored or disposed of safely.
There are problems with producing three dimensional printed circuit boards using these techniques. New techniques are required for this type of printed and plating is not trouble free with these new surfaces and substrate materials. The object of the present invention is to adapt the thermal spray bonding or adherence techniques disclosed in our U.S. Pat. Nos. 4,751,113; 4,521,475; 4,618,504; 4,714,623 (incorporated herein by reference) to forming printed circuit boards and particularly three dimensional printed circuit boards.
According to a preferred embodiment of the present invention, a mixture of resin and hollow micron-sized spheres or beads is applied to the surface of an electronic circuit board by injection molding or by lamination or other composite technology processes. A circuit pattern is etched in the surface by either laser etching, engraving, or other mechanical means such as high speed machining or abrasion. These processes serve to fracture the hollow micron-sized spheres or beads directly under the surface. The fractured surface is cleaned ultrasonically or with high velocity liquid spray to remove surface debris.
The top surface of the circuit board must be without microporosity or surface defects which would serve as anchor sites for sprayed metal. Very fine metal particles may find slight porosity on the surface and will tend to stick to the surface at these points. Normally, they can easily be scraped off the surface and hence do not affect the integrity of the circuit board.
In the preferred embodiment, the circuitry is formed slightly below or flush with the surface.
As an alternative method of preparing a printed circuit board utilizing the thermal spray processes according to the invention, the surface on which the circuit pattern is to be formed is screened with a circuit pattern onto the surface using a resin such as an epoxy. Secondly, the surface is "dusted" with hollow micron-sized spheres or beads in a manner disclosed our U.S. Pat. No. 4,714,623 so that the hollow micron-sized spheres or beads stick to the wet epoxy circuit pattern and are engulfed into the wet circuit pattern until the pattern is saturated with hollow micron-sized spheres or beads. Thirdly, the pattern is allowed to cure then it is lightly abraded to produce the voids, nooks, crannies and undercuts which serve as anchor sites for the sprayed metal. The surface is cleaned to remove any surface debris and, fourthly, the surface is then thermally sprayed with copper or other conductive metal.
In a third process, which somewhat resembles the second process or method and partakes of the process claimed in our U.S. Pat. No. 4,751,113. After silk screening the resin in the desired circuit pattern, and while the resin is still "wet" or uncured, it is thermally sprayed with copper and then the resin is cured. This third method is more dependent on the curing rate of the resin and various parameters such as potlife which determine the appropriate time to spray. Also, the gun distance must be such that the pattern is not distorted by the impingement of the molten particles. Additionally, the type resin used is a factor i.e. whether it's a uv ultraviolet (uv curing resin, a resin that cures only with heat) or a resin that cures with a combination of ultraviolet and heat.
All the above methods of thermally spraying an electronic circuit board depend on a non-porous smooth resin or substrate surface that prevents the sprayed particles from adhering accept where the pattern is established. The overspray is easily collectible and can be reprocessed for respraying whereas acids and other solutions used in the fabrication of other circuit boards are practically totally lost and present problems in storage and disposal.
Considering the large number of steps in fabricating conventional printed circuit boards (as is disclosed in the article referenced above), thermal spraying according to the present invention is a very economical process especially for surface mount boards. Thermal spraying may now be competitive with very fine lines or double sided boards which require plating through holes. In cases where plating through holes is required, inserts or tubular eyelets can be placed into the holes and crimped, and the conductive metal sprayed circuit pattern makes the connection between circuits on opposite sides of the board. In a preferred embodiment, the sprayed deposits should remain or be 2 to 3 mils below the top surface of the board. If the spray deposit fills the groove and starts to become flush with the surface, then the sprayed particles may spread from the edge of the pattern and thereby create rough edges. This may be objectionable for closely spaced combination lines as well for aesthetic or cosmetic reasons.